For years, nautical vessel makers have been aware that placing air bubbles on the bottom of a vessel hull will reduce the drag of the vessel as it passes through water. The challenge as been how to efficiently place the air bubbles on the bottom of the hull. The primary solution to the challenge is to use high energy air compressors to place air on the bottom of a hull for drag reduction. The air placement is typically accomplished by screens or ejector slits mounted in the hull surface of a ship, mostly on the bottom side. Due to the large hull surfaces that need to be covered, large amounts of air need to be provided to the bottom of the ship. Additionally, cleaning means for the screens or ejector slits need to be provided to prevent the clogging generated by marine algae barnacles and other marine organisms. Bottoms with air cavities expose just a fraction of the bottom of the hull to direct contact with the water, thus reducing the drag accordingly.
The concept of using air coated hulls to reduce drag in water has previously been suggested in the maritime literature. Indeed, reducing the hull skin friction component of drag by injecting bubbles or micro-bubbles was first reported in 1973 by the United States Naval Academy using a cylinder coated with small bubbles of hydrogen generated by electrolysis to study reduction in friction. More recently, the United States Defense Advanced Research Projects Agency (DARPA) funded a program to research reduction in friction drag focusing on developing numerical models and computer simulations for air/bubble injection and supported by scale model experiments. In Japan, the National Maritime Research Institute (NMRI) and the Shipbuilding Research Association has carried out bubble experiments using ships and scale models of ships in addition to plate experiments in test tanks. It has been reported that both an effect due to the reduced viscosity of air and the shearing of bubbles in the boundary layer occur. Hull skin friction reductions of up to 5% were reported for ships and up to 80% drag reduction for flat plates. In these experiments, the bubbles were active injections and had a power penalty. Moreover, they were only effective near the point of injection because they did not remain within the boundary layer close to the hull. In the NMRI full-scale tests they also degraded the efficiency of propellers. Another approach pioneered in Russia has been to pump air behind wedge and stepped shaped features to create an air-film along the body of the object, for example a torpedo, or via supercavitation to create the same effect.
It is clear that air films retained at a submerged solid surface should be able to reduce drag, but current approaches require an active input of energy to do so. For example, U.S. Pat. No. 5,524,568, issued to Bobst, discloses a boat hull that “creates a layer or film of bubbles adjacent the submerged region of a boat hull by releasing a flow of air at numerous spaced apart locations on that region of the hull.” However, the Bobst invention requires the use of an air compressor pump, which takes energy and greatly reduces or even completely offsets the energy savings achieved by the effect of the bubbles.
There are numerous patents issued in this field using air bubbles which have been proved in lab tests to lower up to 80% the frictional component of the drag generated by a vessel's motion through water. Due to the fact that the air bubbles are most effective if they are released such that they will wash (or lubricate) the flat hull's bottom, and not released so the air bubble go out and up the side of the hull, the vast majority of these patents explicitly teach or suggest the use of an air compressor. An air compressor is the obvious way to overcome the high static water pressures present at the bottom of the submerged vessel hull. Unfortunately, the standard air compressors, while able to deliver air at high pressure, are very, very inefficient at delivering the high volumes of air needed for covering the large bottom surfaces of a flat bottomed vessel. Moreover, the energy economy obtained by the lubrication is largely offset by the air compressor's fuel consumption, rendering this solution essentially useless.
Other references, such as U.S. Pat. No. 6,748,891, try to replace the compressor using the various methods to create depression where air is drawn, and combine these methods with a standard air fan. The problem with U.S. Pat. No. 6,748,891 and other similar references, is that the small pressure differentials created work only for relatively small drafts. Additionally, regarding the combination with an air fan, there is a tradeoff between the volume and the pressure of the air delivered making them undesirable in applications where both high volume and high pressure are needed. Simply put, the solutions offered by these references do not work beyond a certain draft, and are essentially worthless. Finally, the above suggested solutions are invasive to the hull and expensive to implement on either an existing hull or a newly built hull. The solutions also create additional drag by adding wings outside the originally designed frame of the boat.
Furthermore, there are prior references which disclose an air injector, which is disposed in the stream of water going downwards and under the hull. Full scale experiments with a Japanese cement carrier vessel used precisely these prior art techniques and obtained only a 2-3% drag reduction.
Historically, it has been difficult to convince boat owners to allow their boats to undergo invasive modifications with no guarantee of any gain in efficiency. The bows of the larger carriers, as well as the majority of other types of boats have V-shaped bows that split the stream of incoming water sideways and away from the hull. In this design, the bubbles released in such streams end up mostly on the side of the vessel, with only a small amount washing under the hull. This small amount is typically insufficient to make a difference to the efficiency of the vessel. Even if a large volume of bubbles is released on the bottom of one of these ships in the bow region, these bubbles will quickly wash away and, if not replenished, these bubbles will only lubricate a fraction of the large and typically long hull. As such, the bubbles only lower the drag a small amount.
For smaller crafts, with shallower drafts and relatively short bodies, obtaining sufficient lubrication should be easier to obtain. However, such sufficient lubrication has not yet been achieved in the Prior Art. For example, U.S. Pat. No. 7,004,094 offers a lubrication solution that, when put into practice, is very cumbersome to implement, difficult to maintain, and inoperable. In short, in practice, U.S. Pat. No. 7,004,094 either does not work or does not accomplish a sufficient lubrication for a small vessel.
Thus, what is needed is to provide the equivalent of a bubble layer or an air film in a manner that does not require active power input, or at least, very low power, and which has a strong chance of being retained at the submerged hull surface where it is needed to effectively reduce the drag of the vessel as it moves through the water.
The present invention offers solutions to these problems adapted to be used for either small vessels with a predictable shallow draft and relatively short hulls or large displacement vessels, with variable drafts and long hulls.